In apartment buildings of modern design, when a visitor wishes to be admitted by the occupant of an apartment in such a building, he must stand in front of the closed door of the main entrance and alert the occupant by pressing the appropriate button on the external panel of a device known as an electronic front door announcer; the occupant, thus alerted, may then, from his apartment, by audio or visual means forming part of the aforesaid front door announcer, check the identity of the visitor and, if he decides to admit this visitor, release the automatic opening device of the door of the main entrance by remote control.
The circuits of known electronic front door announcers require a relatively high number of conductors for the common vertical connections and the individual floor connections; this number obviously depends upon the basic design of the announcer and the various functions which it has to perform. Consequently, fitting such an announcer is complex and costly because of the laying of connection sleeves between floors, between the conduit and the various apartments and above all the laying of the various cables and their connections.